Efficient audio sound transduction has a history dating back millions of years. Primitive insect singers generated loud and pure-tone sound with high efficiency by exciting resonators inside their bodies. Male crickets generate chirping sounds via stridulation, where the scraper edge of one wing is rubbed against the ribbed edge of the other wing. Advantageous structural properties of the wings (i.e., relatively large, low-mass flexural membranes) allow efficient muscle-to-sound energy transduction.
In a human context, unnatural (i.e., non-voice) sound production has been explored for millennia, with classic examples being drumheads and whistles for long-range communications and entertainment. In modern society, efficient small-scale audio transduction is ever more important for discrete audio earphones and microphones in portable or wireless electronic communication devices.
For human audibility, an ideal speaker or earphone should generate a constant sound pressure level (SPL) from 20 Hz to 20 kHz, i.e., it should have a flat frequency response. Currently, most commercial speakers are diaphragm based, and most of the diaphragms are driven by a magnetic coil. Because the coil moves together with the diaphragm, the total effective mass becomes large. As a result, the high frequency response may be poor.
To overcome poor high frequency response, acoustic engineers manipulate damping, which basically decreases the response at lower frequencies to make the total response curve flat. It is difficult, however, to engineer an arbitrary damping curve at all frequencies. Further, the damping complicates the acoustic design of the speaker and can increase the fabrication cost significantly. Another problem created by a large effective mass is that it stores kinetic energy which can be released later to jeopardize the music transparency (e.g., the diaphragm does not start or stop immediately with the input signal).